1. Field of the Invention
This invention relates generally to frequency converters and mixers, and relates more particularly to a double-balanced waveguide mixer that utilizes a quad-ridge waveguide and a quad-diode ring.
2. Description of the Relevant Art
A mixer is a non-linear device that converts a low-power signal from one frequency to another by combining it with a high-power signal. Typically, mixers are used to up-convert broadcast signals to higher frequencies for transmission or to down-convert received signals to lower freqencies for signal processing. A mixer receives two input signals, the low-power receive signal (RS) and the high-power local oscillator signal (LO), and produces one output signal, an intermediate frequency signal (IF). The component most prominent in the output signal of a mixer is the product of the two input signals. Assuming that the input signals are sinusoidal functions of time, the output signal will also be a sinusoidal time signal having frequency components at the sum and at the difference of the input signal frequencies. Sums and differences of multiples of the input signal frequencies are also present in the output signal. The component having a frequency equal to the sum of the input signal frequencies is called the upper-sideband signal, while the component having a frequency equal to the difference of the input signal frequencies is called the lower-sideband signal. Usually only one of these frequencies is desired; the other is filtered out in some manner.
Mixers typically utilize diodes to combine the two input signals and generate the output signal. Among the different mixer configurations are single-balanced mixers, which utilize two diodes, and double-balanced mixers, which utilize four diodes. In a double-balanced mixer, the four diodes are configured in a ring structure having four junctions, with each diode coupled between two of the four junctions and with each junctions connecting the anode of one diode to the cathode of an adjacent diode. The receive signal is applied to two opposed junctions of the diode ring, while the local oscillator signal is applied to the other two opposed junctions of the diode ring. In such a mixer, the intermediate frequency signal is conventionally formed by summing the currents at two opposed junctions through the receive signal transformer. Double-balanced mixers have the property of cancelling some of the unwanted harmonics that would otherwise be present in the intermediate frequency signal.
In high frequency communication systems that handle broadcast signals in the K-band, which ranges from about ten to about thirty-five gigahertz (GHz), waveguide transmission lines are typically used to convey signals. Since waveguide mixers have conventionally been difficult to implement, K-band mixers are often implemented using microstrip techniques. In order to use such K-band mixers, the input signals are transferred from waveguides to microstrip lines that feed a single-balanced microstrip or stripline mixer, and the output signal is then retransferred from a microstrip line to a waveguide. This is not a satisfactory solution because of the inefficiencies involved in converting between waveguides and microstrip transmission lines and because of the high cost of building a K-band microstrip or stripline mixer.
Double-balanced K-band mixers have been heretofore unknown due to the difficulties of supplying two input signals having separate grounds to and removing a third signal from the planar structure of the quad-diode ring, and due to fact that the use of transformers to define the intermediate frequency signal is impractical at such frequencies.